I agree on that (although I still mentioned it for the sake of thoroughness).
My background of choice is still the Ubuntu MATE dark (I think that’s what it’s called), but it was one of the original wallpapers from 15.04 or sometime around there. The starry night sky with northern light colours. I hope you know what I mean.
On a fresh install I always do sudo apt-get install ubuntu-mate-wallpaper-* to get them.
Jaymo